It previously has been known to use relatively large detector electrodes to detect large surfaces of a package, in which respect no particularly great requirements are set on the position of the detector electrode relative to the package. However, large contact surfaces between the package and the detector create considerable interference problems. It is also known to use smaller detector elements with a relatively limited detector surface, which necessitates accurate tolerances between the detector elements and the portion of the package to be detected or, alternatively, to provide the detector element with arrangements for displacing the detector according to the dimensions of the package.
Further, GB A 1,399,161 discloses a leakage indicator which requires the package to be submersed into a container of liquid and an electrode to be inserted through the wall of the package. Also, U.S. Pat. No. 3,789,297 shows a pair of electrodes mounted adjacent a seal, requiring both electrodes to contact the electrically conductive fluid for indication.